1. Field of the Invention
The present invention relates to a switching amplifier that performs pulse amplification by using, as a switching control signal, a quantized signal obtained by processing an analog signal or a one-bit signal by delta-sigma modulation.
2. Description of Related Art
A one-bit signal obtained by processing an analog signal or a one-bit signal by delta-sigma modulation is advantageous in that it allows a frequency response to suit an audio source or the like, for example, by widening the effective frequency range or the dynamic range simply through appropriate selection of the constants of an integrator and an adder provided in the delta-sigma modulation circuit that performs the delta-signal modulation. For this reason, new standards for CDs (compact discs) and SACDs (super audio compact discs) adopt a one-bit signal for the recording of an audio signal, and such CDs and SACDs are commercialized. A one-bit signal is used not only for the recording of an audio signal as just mentioned, but also for power amplification and for signal transfer between devices.
A switching amplifier that performs power amplification by using a one-bit signal obtained by delta-sigma modulation obtains a high-voltage switching pulse based on the output of the semiconductor power amplifying device by feeding the one-bit signal obtained by delta-sigma modulation as it is to the control terminal of the semiconductor power amplifying device. Furthermore, it is possible to obtain a power-amplified demodulated analog signal simply by passing this switching pulse through a low-pass filter.
Moreover, since the semiconductor power amplifying device is controlled by the one-bit signal obtained by delta-sigma modulation, it is used in a non-linear region (saturated region) unlike an analog amplifier, which is used in a linear region (unsaturated region). Accordingly, a switching amplifier that performs power amplification by using a one-bit signal obtained by the delta-sigma modulation is advantageously capable of performing power amplification with extremely high efficiency, and such switching amplifiers have been commercialized.
FIG. 7 shows an example of the electrical configuration of a conventional switching amplifier that performs power amplification by using a one-bit signal obtained by delta-sigma modulation (see FIG. 7 of J-P-A-2000-295049).
The switching amplifier shown in FIG. 7 is composed of the following: an input terminal 1; an adder 2; a delta-sigma modulation circuit 3; a pulse amplifier 6 to which a constant voltage is applied from a constant voltage source 7; a low-pass filter 8; an output terminal 9; and an attenuator 10. The delta-sigma modulation circuit 3 is composed of the following: an integrator/adder group 4 provided with a plurality of cascade-connected integrators that integrate one input signal after another and an adder that adds up the outputs of the individual integrators; and a quantizer 5 that quantizes the signal outputted from the adder of the integrator/adder group 4 to convert it into a one-bit signal.
An input signal SIN (an analog signal or a one-bit signal) inputted from an input signal source (not shown) to the input terminal 1 is fed to the adder 2. A feedback signal SFB outputted from the attenuator 10 is also fed to the adder 2. The adder 2 subtracts the feedback signal SFB from the input signal SIN, and feeds the resulting signal to the delta-sigma modulation circuit 3.
The delta-sigma modulation circuit 3 converts the signal fed from the adder 2 into a one-bit signal SQ, and feeds the one-bit signal SQ to the pulse amplifier 6. The pulse amplifier 6 has a switching device (not shown) such as an FET, power-amplifies the one-bit signal SQ by switching the switching device according to the one-bit signal SQ, and feeds the power-amplified one-bit signal to the low-pass filter 8 and to the attenuator 10. The output signal of the pulse amplifier 6 has its high frequency component eliminated by the low-pass filter 8, and is thereby formed into an output signal SOUT that is an analog signal. This output signal SOUT is outputted via the output terminal 9. The output signal of the pulse amplifier 6 is also attenuated by the attenuator 10, and is thereby formed into the feedback signal SFB.
The pulse amplifier 6, which power-amplifies the one-bit signal SQ, produces switching loss attributable to the capacitive and resistive components of the switching device it includes. This switching loss lowers the power efficiency of the switching amplifier shown in FIG. 7.
Hence, from the viewpoint of energy saving, measures are sometimes taken to reduce the number of times that the switching device switches in the pulse amplifier 6 with a view to reducing the switching loss. In the switching amplifier shown in FIG. 7, so as to reduce the switching loss, the number of times that the switching device switches in the pulse amplifier 6 is reduced by lowering the sampling frequency of the delta-sigma modulation circuit 3.
However, a low sampling frequency of the delta-sigma modulation circuit 3 disadvantageously narrows the noise shaping frequency range or when the input signal SIN is an analog audio signal, disadvantageously reduces the SN ratio (signal-to-noise ratio) of the audio frequency range with respect to which the analog audio signal is processed.
On the other hand, from the viewpoint of a wider dynamic range and low residual noise, measures are sometimes taken to raise the sampling frequency of the delta-sigma modulation circuit 3 in the switching amplifier shown in FIG. 7 with a view to increasing the amount of information per unit time.
So as to raise the sampling frequency of the delta-sigma modulation circuit 3, however, the master clock frequency needs to be raised, and this increases radiated noise. Hence, extra cost for measures against EMI (electromagnetic interference) is required, which is disadvantageous.